1. Field of the Invention
The present invention generally relates to network negotiations executed at the beginning of a two-way session, and more particularly to a terminal device, a data transmission-reception system and a data transmission-reception initiation method, in all of which the optimum communication mode is determined by notifying predetermined capability information on the terminal device.
2. Description of the Related Art
Widening bandwidths of networks and the increased prevalence of Internet appliances are beginning to create an environment in which communications such as two-way video telephony can be performed easily.
A technique using the Session Initiation Protocol (SIP) proposed as Request For Comments (RFC) 2543 is gradually becoming mainstream as a technique for initiating a two-way session such as Internet telephony.
The SIP is a protocol for handling call signaling and control in communication services based on the Internet Protocol (IP). The standardization of the SIP is being advanced by the Internet Engineering Task Force (IETF). Using the SIP makes it possible to initiate, alter and terminate two-way communications. The kinds of communications which can be dealt with by the SIP are not limited to communications via voice, but go so far as to encompass video communications, text based chat (communicating via text), interactive video communication games and virtual reality.
The SIP initiates and terminates a session, but is not concerned with the content of the session. Accordingly, as to the content of the session, the Session Description Protocol (SDP), which was proposed as RFC 2327, describes information on the session and media formats which can be used. Then, the SIP transmits the SDP describing such information to a communication partner as an INVITE message (a request to participate in the session), and initiates the session by way of a negotiation with the communication partner. In the context above, media format refers to, for example, formats such as the Joint Photographic Experts Group (JPEG) and Moving Picture Experts Group (MPEG), for example, where the medium is image data or the like.